I'm propably not telling anything new to veteran pixelartist, but since I've been experimenting a bit with colours in the last few days, I thought to share my findings.
Some of the newer mainline sprites (eg. Firedragon, mudcrawler, elvish outrider) use a lot of shades you (at first) wouldn't expect.
For example the Firedragon has purple and greenish hues, while the elvish armor consists of yellow and blue colours.
Additionally some shades can be used for very different surfaces.
Here I made a white dress with the same colours as the elvish armor (2 blueish shades, bright yellow and white)
The whole fairy is made out of only 12 colours (not counting magenta). Hair,skin,dress,shoes and staff reuse the same shades with various different effects.
I was trying to make a palette, which would look good on many different sprites. Though I only partially succeded, I've put togehter this 36 coulour palette (including 5 magenta hues, shadow, and transparent background). It is far from perfect. Some shades will hardly be useful, while many better ones are still missing, but I think it's a good start.
Finally, here are some changes I made to my units, to show you the effect of the palette.
The top-right one has only 13 colours (around 18 with magenta and shadow), while the old one had about 30 (including TC). As you might see (or not), the armor looks hardly different (from afar), even though I completly changed the colours from grey to green,brown and blue. All shades are included in the palette.
The bottom right one was a bit unfortunate. The brown shirt couldn't be replaced with colours from the palette, so I only made changes to hood, weapons and skin. As a sidenote: the face is a bit greyish and sickly, but this was a pure artistically choice; the palette inclued several shades for a pinker skin.
It is arguably which sprites look better; the point of this was more to see the effect different colours can have on a image.
Especially darker shades can be changed (to green or purple) to give the sprite a nice undertone without changing to much.
Lighter shades can look better with yellow or blue.
Shadows on a surface might look better when you use another colour insted of just a darker hue. Here are some good pairings (imo): Red/brown/orange -> purple shadow
Grey/black -> blue/green shadow
Gold/yellow -> red/brown shadow
TL;DR: Colours can be changed